This is a study of kinetics of permeability of certain biomembranes and their modification by certain agents, in particular, ouabain and peptide hormones, especially insulin. Membranes tested are sarcolemma and sarcoplasmic reticulum from skeletal muscle and plasma membranes from adipocytes. The hypothesis is that major control of membrane properties, and, through these, of many cell properties, occurs at the interface between membrane and external phase, that changes in configuration (and function) of protein components of membranes occur in response to information emanating from association between regulation agent (i.e., insulin) and specific membrane site. Extent of these changes will be investigated by correlations of altered optical properties, mainly using fluorescent probes, with altered permeability, transport, and electrical properties.